The present invention relates to uninterruptable power supplies and in particular to a method and apparatus for assessing a parameter of the cells in the batteries of such power supplies.
Uninterruptable power supply systems are used in situations where unexpected loss of power is particularly undesirable, for example by financial institutions, telecommunications installations, the utilities, hospitals and the military. They are needed where loss of power is unacceptable, for example where hospital patients rely on life support systems, or where data loss due to a computer shut down would be unacceptable as in a financial institution. The battery of the uninterruptable power supply is typically the last line of defence against total shutdown during power outages.
A typical arrangement of an uninterruptable power supply is shown in FIG. 1. An external alternating current (a.c.) power supply 1, generally supplied by an outside utility company, is converted to direct current (d.c.) by a rectifier 2. The rectified d.c. is converted back to a.c. by an inverter 3 for supply to the power supply user 4. A battery 5 is connected to the d.c. part of the system in such a way that the charge on the battery is maintained during normal operation of the power supply. The battery may typically comprise a large number of lead acid cells. Should the external power supply 1 fail for any reason, the battery 5 maintains the operating voltage of the d.c. part of the system so that the power supply to the user 4 is maintained.
Battery 5 is shown, for convenience, and simplicity connected between the D.C. voltage and earth. In practise, however, especially on larger batteries, the battery is generally at a voltage floating with respect to earth, as supplied by the rectifiers.
Unless the battery is healthy, it may not be able to carry the required electrical load when the a.c. supply is cut off. Thus it is desirable to be able to determine an indication of the condition of the cells in the battery, so as to be able to take further action, for example by repairing or replacing a cell, if a cell is unlikely to be able to meet the power demands whilst the a.c. supply is cut off.
Batteries are generally manufactured with a certain life span that is dependent on environmental criteria and the number of discharges supplied by the battery. Some of the discharges will be due to use of the battery during a.c. power cuts, but some may occur during load testing. One typical method of determining battery health employs a load test. During this load test, the battery is disconnected from the power supply system and discharged across a load such as a resistor bank. The rate at which the cell voltage then decays is indicative of the battery""s health and ability to sustain the power supply should the a.c. supply be cut. Weak battery cells display earlier and more rapid signs of voltage decay. The voltage decay characteristic obtained during a load test correlates well with the expected performance, but the test is labour intensive and cannot easily be performed with the battery connected to the operating uninterruptable power supply. Furthermore, battery lifetime is reduced as a result of the required discharge.
To prolong battery lifetime, therefore, modes of testing that do not involve large discharges have been developed. For example, reducing the depth of discharge during battery testing greatly improves battery lifetime. An alternative to load testing is to use impedance measurements to determine battery condition.
Any device through which an electrical current will flow exhibits an impedance to that flow. In a lead acid battery the impedance comprises pure resistance components such as the battery terminals, plates, and the resistance of the electrochemical path, and capacitative components, in particular of the parallel plates. The impedance of the battery will therefore depend on the frequency at which it is measured. Detailed analysis of battery impedance measurements is difficult, requiring complex calculations. No universal equivalent circuit is available to describe the response of even a single electrode. The situation is yet more complicated when considering complete cells or batteries, with the influence on impedance of all the individual components being difficult or impossible to separate. For this reason, battery impedance measurements in practice are usually limited to one or a few impedance measurements at fixed frequencies. Deviations of a single cell from a norm may then indicate that this cell is faulty.
Although the battery resistance can be measured using a d.c discharge across two or more different loads, battery lifetime may be affected by the significant discharge required to obtain repeatable readings, and a long measurement cycle is needed to ensure that battery recovers before taking measurements from the next cell. These problems do not occur during an a.c. impedance measurement. A variety of frequencies have been suggested or used for such measurements, ranging from 10 Hz to 1 mHz. A signal generator is used to apply an a.c. signal of the required frequency to individual cells or to the whole battery. Current and voltage readings are then made to determine the impedance of individual cells.
By use of an on-line monitor it is possible to look for changing cell float voltages and cell impedance values that signal that the characteristics of the cell are changing. In such an application it does not matter that a physically correct value of impedance is not returned by the monitor. Rather, the monitor needs to determine whether a measure of the impedance of a given cell or group of cells has changed significantly over time, perhaps with respect to some baseline or norm, or whether the measure of impedance of one cell or group of cells is significantly different from the battery average. Long term stability is an important indicator of cell performance and health. The skilled and experienced person is able to make a decision to make further checks on a cell or group of cells, perform repairs or install a replacement, based on the measurements of impedance. Automatic monitoring equipment, perhaps embodied in a computer connected to a telecommunications link, may be used to assist in this process.
Impedance monitors of the prior art have measured cell impedance by injecting an a.c. signal of a given frequency into the battery and by filtering measurements of voltage and current at the same frequency. Because of the large capacity of the batteries used in uninterruptable power supplies the signal generator may need to be of a considerable size.
It is an object of the present invention to provide an improved method for determining the condition of cells within the batteries of uninterruptable power supplies.
According co the present invention, there is provided a method of measuring the electrical efficacy of one or more battery cells for use in an uninterruptable power supply, the method comprising: measuring at least one of an a.c. component of a current through the battery cell or cells and an a.c. component of a voltage across the said battery cell or cells, the a.c. component arising from a ripple current in the said battery cell or cells in use; and determining the electrical efficacy of the cell or cells on the basis of the or at least one of the measured a.c. current and voltage components.
The ripple current in the battery results from the normal operation of the uninterruptable power supply. In particular, it may result from the operation of those components converting between direct and alternating current. Electrical signals related to the ripple current include the ripple current itself and ripple voltages driving or driven by the ripple current.
Preferably, the step of determining the electrical efficacy includes obtaining a numerical value from the, or at least one of the, measured a.c. current and voltage components.
In that case, the electrical efficacy of the or each battery cell may be determined by comparison of the said numerical value with a corresponding further numerical value obtained by measurement of a.c. current and/or voltage components from one or more different cells. The electrical efficacy may in a particularly preferred embodiment be determined by comparison of the said numerical value with the average of a plurality of further numerical values obtained by measurement of a.c. current and/or voltage components from a corresponding plurality of separate arrays of single or multiple cells respectively.
Alternatively, the electrical efficacy of the battery cell or cells may be determined by comparison of the said numerical value with a corresponding predetermined numerical value.
The method may further comprise the steps of measuring both the a.c. component of current through the battery cell or cells and the a.c. component of the voltage across the battery cell or cells; and obtaining a value for the internal impedance of the battery cell or cells via a combination of the said current component and the said voltage component.
Although the impedance of the cell or cells is measured in preference, other parameters may be conveniently assessed, as will be apparent to the person skilled in the art. For example the resistive, capacitative or inductive components of the battery impedance, or the power dissipated in the cell or group of cells within a given frequency band may be determined. These and other parameters may be of use in assessing the condition of the cell or group of cells.
Preferably, the step of measuring at least one of the a.c. components of a current and a voltage includes the steps of: measuring electrical signals representative of at least one of the voltage level across the cell or cells and the current level through the cell or cells; and frequency filtering the or each measured electrical signal to extract the said a.c. component arising from the ripple current. In that case, the step of filtering includes isolating a band of frequencies from the or each said electrical signals.
The band of frequencies may include at least one harmonic frequency of the a.c. mains frequency, such as harmonics of 50 Hz or 60 Hz. Most preferably, components at 900 Hz and 1080 Hz are chosen.
The invention also extends to an apparatus for measuring the electrical efficacy of one or more battery cells for use in an uninterruptable power supply, the apparatus comprising an ammeter arranged to measure an a.c. component of a current through the battery cell or cells, the a.c. current component arising from a ripple current in the said battery cell or cells in use, the electrical efficacy of the cell or cells being determined on the basis of the measured a.c. current component.
In yet a further aspect, the invention resides in an apparatus for measuring the electrical efficacy of one or more battery cells for use in an uninterruptable power supply, the apparatus comprising a voltmeter arranged to measure an a.c. component of a voltage across the battery cell or cells, the a.c. voltage component arising from a ripple current in the said battery cell or cells in use, the electrical efficacy of the cell or cells being determined on the basis of the measured a.c. voltage component.
In that case, the apparatus may further comprise an ammeter arranged to measure an a.c. component of a current flowing through the battery cell or cells, the a.c. current component also arising from the said ripple current in the said battery cell or cells in use, the electrical efficacy of the cell or cells being determined on the basis of both the measured a.c. voltage component and the a.c. current component.
A filter such as a fifth order band pass filter may also be provided to isolate harmonic frequencies of mains frequencies, for example.
Advantageously, apparatus according to the present invention may be characterised in that it does not comprise a signal generator for injecting a current into the battery.